Sure Santos was a black belt in BJJ they give those belts out like candy now a days. He's not exactly know for his submission skills as a fighter ( only has a couple wins by submission) I would not say Diaz beat him at his own game by going to the ground with Cyborg. Cyborg was know as more of a striker then a grappler at the time Cyborg was having success with leg kicks in the match had he kept it standing the fight may have ended differently.
He beat an undersized Penn by using his reach and size to clinch Penn up against the fence. I've already conceeded that Diaz has good boxing. In this case he used his strength and size to beat and undersized opponent. One of the few times Diaz actually used a gameplan.
And that kind of goes to my point once faced with elite level guys he lost. He had what I consider to be an easy run while in Strikeforce against opponents he matched up favourably with. I'm not saying Diaz doesn't have talent or skills or even put on entertaining fights, but look at his career and what is the most impressive win he has ?someone who did that well at beating people at their own game is not mediocre. the only people he has lost to in the past 5 or 6 years are gsp and condit. there is no same in losing to either of those guys. gsp is the GOAT by far in that division and condit is a beast as well plus that was aclose fight.